misc(rmp): allow use of newer byteorder dep
misc(rmp): allow use of newer byteorder dep

Right now byteorder 0.4.2 works fine without code changes.

Switching to the newer versions allow users of this crate to avoid
pulling in (unneededly) multiple versions of the same crate (byteorder
uses libc, which is widely depended on)

3Hren/msgpack-rust